This invention relates to a counterflow heat exchanger having two fixed tube plates and a thermal exchange region comprising substantially straight tubes, and in particular to a heat exchanger for high pressure high temperature operation suitable for use in conventional and nuclear power stations as well as in other industrial plants.
As is known, many types of industrial plants utilize counterflow heat exchangers which are of considerably large size and, owing to the severe service conditions, to provide reliability of the highest degree, both to avoid the necessity of stopping the plant, which obviously affects the operation economy, and for safety reasons, e.g. steam generators using sodium as the primary fluid which equip nuclear plants of the LMFBR type.
In a majority of cases, such heat exchangers currently comprise a pair of tube plates facing each other and spaced apart from each other, which are interconnected by a tube nest which is welded to the plates, in a manner that will be described hereinafter, for the passage of the secondary fluid therethrough. Also provided is an outer shroud which unites the plates together and encloses the tube nest to confine the region of primary fluid passage. The construction of these exchangers, as well as that of other conventional designs, has first of all the disadvantage--specially evident in the cited steam generators using sodium as the primary fluid--of a non-uniform primary fluid flow at the thermal exchange region, which primary fluid flow is faster at the central portion of its cross-section than at the periphery: this leads to a non-uniform distribution of the wall temperature in the various tubes, with attendant negative consequences, in particular of a mechanical and constructional nature, as the expert will readily recognize.
The primary fluid flow is also imperfectly uniform at the inlet and outlet ends, which comprise conventional annular headers or manifolds, usually located externally to the tube nest.
Known is that in exchangers affording high reliability levels, the best method currently in use for welding the tubes to the tube plates is included among those known as IBW (Internal Bore Welding) procedure, and consists of carrying out a butt weld process in which the tubes are welded to tailpieces purposely formed on the plates with a bore substantially equal to the inside bore of the tubes; more specifically, this type of weld, known per se, contemplates seating of the tube end to be welded in a seat formed on the tailpiece previously formed on the plate, thereafter access is gained, with a welding torch, to the inside of the tube, at the junction area, to perform a weld which usually involves no addition of material.
This type of weld, especially on account of the severe operating conditions of the exchanger types mentioned above, must be then checked individually, generally by X-ray, to ascertain the reliability thereof. Now, in a majority of the conventional heat exchangers, as mentioned above, the tube plates are arranged to face each other at a distance apart, thereby it is necessary, for permitting insertion of the individual tubes constituting the nest in conformity with the necessary manufacturing and control procedures, to provide in one plate bores of substantially the same size as the tube outside diameter, whereas on the other plate it is possible to prearrange the tailpieces with seats adapted for accomodating the type of weld just described.
A serious drawback with conventional exchangers is that bores must be formed in a plate which have the same size, or slightly larger size, as the tube outside diameter, thereby it becomes necessary to hold the tube end a few millimeters inside the bore, suitably made oversize to permit the tube passage; consequently when the weld is performed of this end of the tube to the tube plate, the weld area is not rectilinear but is of necessity forced to assume a flared shape of substantially truncated cone pattern.
The presence of this flare at the weld areas has first of all the drawback of undergoing flex and shear actions, technically undesired in this type of joint, and moreover this type of weld is difficult to radiograph, thereby considerable problems are created for the control step; technical problems may also originate from the fluid dynamic characteristics involved.
Another serious drawback of the heat exchangers of the type mentioned above is that, for obvious construction and control reasons, at least part of the outer shroud must be applied to the plates after welding the tube nest of the plates, thereby considerable difficultly is encountered in radiographing the shroud welds, while it is impossible to reweld, owing to the lack of accessibility from the inside. It should be added to the above that connecting the shroud to the tube plates after the application of the tube nest is an operation which makes the heat treatment of the welded areas of the shroud extremely difficult to carry out.
In the heat exchangers of the type just described an expansion joint may be provided on the shround or skirt to allow for thermal expansion differentials between the tube nest and shroud; although in this case there exists a chance of the whole tube nest expanding, any expansion differentials between single tubes are prevented, as may result from various causes, among which a different distribution of the flow from one tube to the other. These expansion differentials unavoidably generate tensions which concentrate at the weld areas between the tubes and tube plates, with obvious danger and inconvenience, mainly at the conventional joints, wherein as described flex and shear efforts are generated.
A further drawback of almost all of the known types, and specially felt when high pressures are involved, resides in that the forged stock used in forming the tube plates has a considerably high mass, with all its attendant complications of a mechanical, thermal and metallurgical nature; also substantial are the difficulties of assembling the tube nest.